This invention relates to an active matrix substrate of an active matrix liquid crystal display and a method of producing the active matrix substrate.
The use of active matrix liquid crystal displays is rapidly expanding. In most of the current active matrix liquid crystal displays the switching device for each pixel is a thin-film transistor (TFT) using a semiconductor film. In most cases the semiconductor is amorphous silicon, but in some cases polycrystalline silicon or a compound semiconductor such as CdSe is used.
FIGS. 13 and 14 of the accompanying drawings show the construction of a coventional active matrix substrate of a liquid crystal display having a TFT for each pixel. On a transparent insulator substrate 50 such as a glass substrate, a gate electrode 52 and a gate bus line 54 are formed by patterning a metal film such as a chromium film. The electrode 52 and the bus line 54 are buried in a transparent dielectric film 56 such as a silicon nitride film. On the dielectric film 56, an island structure of a semiconductor film 58 such as an amorphous silicon film and an n-type semiconductor film 60 such as a phosphorus-doped amorphous silicon film is formed above the gate electrode 52 to provide a TFT. A drain electrode 62, a drain bus line 64 and a source electrode 66 are formed by patterning a metal film such as a chromium film or an aluminum film. A pixel electrode 70, which is in contact with the source electrode 66, is formed by patterning a transparent conductor film such as an indium-tin oxide (ITO) film. The gate bus line 54 and the drain bus line 64 extend perpendicular to each other, but these two bus lines must not intersect each other. So, the two bus Lines 56 and 64 are formed in two different layers.
In producing this structure, five photolithography steps are needed. The first photolithography is for forming the gate electrode 52 and gate bus line 54. The second is for forming the island structure of the semiconductor films 58, 60. The third is for forming contact holes in the dielectric film 56 to provide access to the gate bus line. The fourth is for forming the drain electrode 62, drain bus line 64 and the source electrode 66. The fifth is for forming the transparent pixel electrode 70.
If the gate bus line and the drain bus line can be formed in the same layer, there arises a possibility of decreasing the total number of photolithography steps and consequently reducing the production cost. In this connection, JP-A 60-128486 (1985) proposes the structure shown in FIGS. 15-16 of the accompanying drawings.
The active matrix substrate in FIGS. 15-17 differs from the active matrix substrate in FIGS. 13-14 in the following points. The gate bus line 52 is formed on the dielectric film 56. A contact hole 72 is used to connect the gate bus line 54 to the gate electrode 52 under the dielectric film 56. The drain bus line 64 on the dielectric film 56 is broken to provide a gap 65 so that the gate bus line 54 extends across the gap 58 without intersecting the drain bus line 64. Under the gap 65, a bridge electrode 76 is formed on the glass substrate 50 under the dielectric film 56, and the drain bus line 64 on both sides of the gate bus line 54 is connected to the bridge electrode 76 by using contact holes 74. In this structure the gate bus line 54 and the drain bus line 64 are formed in the same layer by patterning a metal film. However, the production of this structure still entails four photolithography steps: the first for forming the gate electrode 52 and the bridge electrode 76, the second for forming the island structure of the semiconductor films 58, 60, the third for forming the contact holes 72, 74 and the fourth for forming the drain electrode 62, drain bus line 64, source electrode 66 and the pixel electrode 70. Besides, this structure is not suited to large area displays because the total resistance of bus lines, viz. wiring resistance, greatly increases for the following reason. In this case the bus lines 54, 64 are formed of a transparent conductor such as ITO since the bus lines 54, 64 and the pixel electrode 70 are formed simultaneously. Conventional transparent conductor films are greatly higher in resistivity than metal films. For example, volume resistivity of ITO film is about 20 times as high as that of chromium film.